Hermetic packaging

ABSTRACT

A method of hermetically packaging an electronic device ( 8 ), in an enclosure ( 2 ) comprising mutually inter-engageable first and second housing members ( 4, 6 ), comprising the steps of securing the electronic device ( 8 ) to the first housing member ( 4 ), engaging the first ( 4 ) and second ( 6 ) housing members such that an hermetic seal is provided there between, wherein the engagement step is performed in a controlled atmosphere. The hermetic seal may be provided by an interference fit between the first ( 4 ) and second ( 6 ) housing members or via sealing means ( 16 ) interposed between the housing members ( 4, 6 ). The second housing member ( 6 ) may comprise an optical element ( 20 ), for example a window or lens. The packaging method is particularly applicable to packaging thermal detectors, for example microbolometer arrays.

The present invention relates to hermetic packaging and in particular toan hermetically sealed enclosure and a method for forming saidenclosure. The invention has particular relevance to hermetically sealedenclosures for electronic devices.

Many electronic devices must be packaged in an hermetic (airtight)enclosure in order to maintain correct operation. Furthermore, in manycases, there is a requirement for the package to be evacuated to a lowpressure. Examples of such vacuum packaged devices include movablemicromirror arrays, micromachined inertial sensors (accelerometers andgyroscopes), microbolometer arrays, gas sensors and absolute pressuresensors.

Hermetic packaging requires that a sealing step be performed under acontrolled atmosphere, usually of an inert gas such as nitrogen orargon. As mentioned above, there may be an additional requirement forthe package to be evacuated to a low pressure.

Outgassing during the sealing processes can introduce contaminants intothe controlled atmosphere within the enclosure, which can be ofparticular concern for high purity hermetic enclosures. In many casesthe sealing process itself can be a significant source of outgassing,which degrades the vacuum level achieved and can introduce a variety ofcontaminants. Moreover, outgassing is invariably exacerbated if thesealing process includes a heating process.

Conventional sealing processes are often performed at elevatedtemperatures. This is often the highest temperature the device canwithstand and hence is a higher temperature than the device hasencountered elsewhere in the bake-out process. This leads to outgassingoccurring from all elements of the package and during the sealingprocess, hence degrading the vacuum level and introducing contaminantsinto the atmosphere.

An additional problem of sealing at elevated temperatures is the stressformed in the seal during cooling due to the differential thermalexpansion coefficients of the materials involved.

Automation of such sealing processes can also be complex and expensivedue to the requirements for vacuum, temperature control, alignment andother processing parameters. Alternative sealing methods in which theinside of an enclosure is evacuated and then the enclosure is sealed byclosure of a valve or exhaust tube require additional parts andprocessing steps for the sealing process and tend to be subject toeither poor hermeticity, high cost or outgassing during a heatingprocess in the sealing step.

An alternative sealing method which does not require a high temperaturesealing step is described in U.S. Pat. No. 6,252,229. U.S. Pat. No.6,252,229 describes a sealed cavity microstructure having first andsecond wafers that are positioned relative to each other so as to from acavity there-between. The microstructure includes a pressure seal and astructural bond arranged between the first and second wafers tointegrate said wafers.

The technique of U.S. Pat. No. 6,252,229 utilises the accuratedimensions and close tolerances achievable with wafer scale processingmethods to provide a reliable seal to the microstructure.Notwithstanding the effectiveness of this technique, the resultingsealed cavity exhibits a small internal volume, making any vacuumcontained therein vulnerable to compromise during the lifetime of thedevice.

It is an object of the present invention to mitigate at least some ofthe disadvantages of the above packaging methods, and to provide asimplified hermetic packaging method and hermetically sealed enclosure.It is a further object of the present invention to provide an hermeticpackaging method in which the temperature of the heating process isminimised or which obviates heating.

According to a first aspect of the present invention, a method ofhermetically packaging an electronic device, in an enclosure comprisingmutually inter-engageable first and second housing members, comprisesthe steps of

(i) securing the electronic device to the first housing member,

(ii) engaging the first and second housing members such that an hermeticseal is provided there between,

wherein the engagement step is performed in a controlled atmosphere.

The method provides a simple and cost effective sealing procedure forhermetically packaging the electronic device. Furthermore, the sealingstep reduces contamination of the controlled atmosphere and reducesoutgassing. Where the controlled atmosphere comprises a vacuum,degradation of the vacuum during sealing is reduced.

In a preferred embodiment, the first housing member comprises a baseportion, to which the electronic device is secured during the securingstep, and an engagement portion adapted to engage with the secondhousing member, and wherein the method further comprises the step ofattaching the base portion to the engagement portion prior to engagingthe first and second housing members.

The step of attaching the base portion of the first housing member tothe engagement portion does not need to be performed in an inertatmosphere or a vacuum, as it would be in a conventional vacuum assemblyprocess. Accordingly, a wider range of processes can be used to packagethe electronic device and hence the assembly cost can be reduced.

In another preferred embodiment, the first and second housing membersare adapted to inter-engage to form an interference fit there between,said interference fit providing the hermetic seal.

Alternatively, the enclosure further comprises sealing means interposedbetween the first and second housing members, said sealing meansproviding the hermetic seal.

The sealing means may comprise at least one of a metal, a eutecticalloy, an elastomer and an adhesive. For example, the sealing meanscomprise an indium seal or a compressible elastomeric ring.

Conveniently, the method further comprises the intermediate step ofapplying the sealing means to least one of the first and second housingmembers prior to engaging the first and second housing members.

Advantageously, the enclosure further comprises spacer means disposedadjacent the sealing means so as to preclude over-compression of thesealing means during the engagement step. Additionally, oralternatively, the enclosure comprises retaining means disposed adjacentthe sealing means so as to retain the sealing means.

In a preferred embodiment, the second housing member comprises a firstsubstantially transmissive optical element and an engagement portionadapted to engage with the first housing member.

Where enclosure comprises sealing means, the hermetic seal may beprovided between the first housing member and the first optical elementvia said sealing means.

In another preferred embodiment, the second housing member is adapted toreceive a second substantially transmissive optical element.

Advantageously, the controlled atmosphere comprises an inert gas. Theinert gas may comprise at least one of nitrogen and argon.

Preferably, the controlled atmosphere comprises a vacuum.

The step of engaging the first and second housing members may includethe step of bonding said housing members. The bonding step may compriseone of friction welding and friction soldering.

Conveniently, the first and second housing members comprise metalcylinders having a substantially circular cross section.

According to a second aspect of the present invention there is nowproposed an electronic device comprising an electronic element, a firsthousing member, and a second housing member, the first and secondhousing members having an engagement hermetic seal there between so asto define around the electronic element an hermetic enclosure having acontrolled atmosphere within.

According to a third aspect of the present invention there is nowproposed an electronic device comprising an electronic element, a firsthousing member, and a second housing member, the first and secondhousing members defining an hermetic enclosure,

wherein the electronic element is located within the hermetic enclosureand wherein the hermetic enclosure is formed by engaging the first andsecond housing members in a controlled atmosphere such that anengagement hermetic seal is provided there between.

The above electronic devices are advantageous in that they utilise asimple and cost effective hermetic seal. Furthermore, the hermetic sealreduces contamination of the controlled atmosphere and reducesoutgassing during the sealing process. Where the controlled atmospherecomprises a vacuum, degradation of the vacuum during sealing is reduced.

In a preferred embodiment of the second or third aspect of the presentinvention, the engagement hermetic seal comprises an interference sealbetween the first and second housing members.

In another embodiment of the second or third aspect of the presentinvention, the engagement hermetic seal comprises a friction weldbetween the first and second housing members.

Preferably, the electronic device according to the second or thirdaspect of the present invention further comprises sealing meansinterposed between the first and second housing members, said sealingmeans providing the engagement hermetic seal. Conveniently, the firstand second housing members may be held in engagement by an interferencefit there between.

In a preferred embodiment of the second or third aspect of the presentinvention, the sealing means comprise at least one of a metal, aeutectic alloy, an elastomer and an adhesive. For example, the sealingmeans comprise an indium seal or a compressible elastomeric ring.

In another preferred embodiment of the second or third aspect of thepresent invention, the enclosure further comprises spacer means disposedadjacent the sealing means so as to preclude over-compression of thesealing means. The enclosure may comprise retaining means disposedadjacent the sealing means so as to retain the sealing means.

In a further preferred embodiment of the second or third aspect of thepresent invention, the second housing member comprises a firstsubstantially transmissive optical element and an engagement portionadapted to engage with the first housing member.

The first optical element may comprise a lens. Advantageously, the firstoptical element comprises at least one of chalcogenide glass, siliconand germanium.

Where the electronic device according to the second or third aspect ofthe present invention comprising sealing means, the engagement hermeticseal may be provided between the first housing member and the firstoptical element via the sealing means.

In another preferred embodiment of the second or third aspects of thepresent invention, the second housing member includes a secondsubstantially transmissive optical element. The second optical elementmay comprises chalcogenide glass.

In a further preferred embodiment of the second or third aspects of thepresent invention, the controlled atmosphere comprises an inert gas. Theinert gas may comprise at least one of nitrogen and argon. Preferably,the controlled atmosphere comprises a vacuum.

According to a fourth aspect of the present invention, there is nowproposed a thermal detector housed within an hermetic enclosurecomprising mutually inter-engaged first and second housing members, thesecond housing member comprising a first substantially transmissiveoptical element; wherein said housing members enclose the thermaldetector within a controlled atmosphere and provide an hermetic sealaround said thermal detector.

In a preferred embodiment, the first and second housing members form aninterference fit there between, said interference fit providing thehermetic seal.

In another preferred embodiment, the enclosure further comprises sealingmeans interposed between the first and second housing members, saidsealing means providing the hermetic seal.

Conveniently, the sealing means comprise at least one of a metal, aeutectic alloy, an elastomer and an adhesive. For example, sealing meansmay comprise an indium seal or a compressible elastomeric ring.

Advantageously, the first optical element comprises a lens. The firstoptical element may comprises at least one of chalcogenide glass,silicon and germanium.

Where the enclosure comprises sealing means, the hermetic seal may beprovided between the first housing member and the first optical elementvia the sealing means.

In a further preferred embodiment, the second housing member includes asecond substantially transmissive optical element. Advantageously, thesecond optical element comprises chalcogenide glass.

In a preferred embodiment, the controlled atmosphere comprises an inertgas. The inert gas may comprise at least one of nitrogen and argon.

Advantageously, the controlled atmosphere comprises a vacuum.

The invention will now be described, by example only, with reference tothe accompanying drawings in which;

FIG. 1 shows a schematic cross sectional representation of an hermeticenclosure according to the present invention,

FIG. 2 shows a cut-away schematic representation of an hermeticenclosure according to the present invention,

FIG. 3 shows an exploded schematic representation of an hermeticenclosure according to the present invention,

FIG. 4 shows a schematic cross sectional representation of analternative hermetic enclosure according to the present invention.

Referring to FIG. 1, an hermetically sealed enclosure (2) according toone embodiment of the present invention comprises a first housing member(4) and a second housing member (6) which together enclose an electronicdevice (8) disposed within said housing members. The housing members (4,6) are arranged such that they fit together to form an hermetic sealaround the electronic device. In practice the housing members (4, 6) maycomprise inter-engageable cylinders. In FIG. 1, the second housingmember (6) has a closed end to provide a lid for the hermetic enclosure.FIG. 1 shows the first housing member (4) attached to a separate base(10). However, a separate base is not required if the first housingmember (4) is closed cylinder, in which case the base (10) is integralto the first housing member (4). In practice, the cylinders may befabricated from metal, ceramic, glass, plastic or any other suitablematerial. In particular, the cylinders may exhibit a substantiallycircular cross section.

The electronic device (8) is disposed on a base (10) and connected toexternal connections (12) via feedthroughs (14) on the base (10). Thefeedthroughs (14) may comprise conventional glass-to-metal feedthroughs.

The first and second housing members (4, 6) may be arranged such thatthey fit together to form an interference fit, thereby providing anhermetic seal around the electronic device. Alternatively, or inaddition, a seal (16) may be disposed between the first housing member(4) and the second housing member (6). The seal (16) provides acontinuous, airtight connection between the two housing members (4, 6).

In practice, the seal (16) may be a ring of soft metal such as indiumlocated on the top surface of the first housing member (4). The seal(16) may be accompanied by one or more additional ring (18) which actsas a spacer layer to prevent over-compression of the seal. Additionally,or alternatively, the ring (18) may act as a dam to prevent the escapeof the sealing material (16).

The additional ring (18) which acts as the spacer and/or dam featurescan also be designed into the shape of the housing member (4, 6), thusreducing the number of components and assembly steps. Location featuresmay also be included to ease assembly.

Other sealing materials can be used, for example other soft metals,solders and eutectic alloys, as well as rubber seals and glues. Forexample, an adhesive could be applied outside of the seal (16) to addmechanical strength to the seal (16). In principle, the seal materialcould be added in vacuum, for example by vacuum evaporation. The sealingring (16) could also be deposited onto the assembly by an overmouldingprocess in the case of an elastomeric seal.

The hermetic enclosure is fabricated as follows. The electronic device(8) is attached to the base (10) (for example, a transistor can headersuch as a TO-5 or a TO-8 header) by industry standard processes. Anyconventional low-outgassing die attach methods may be used. If required,the electrical connections are then bonded to the feedthroughs (14) (forexample, glass-to-metal feedthroughs) on the base (10), also in astandard manner. The first housing member (4), comprising a metalcylinder in this instance, is attached to the base (10). The metalcylinder may be formed by drawing, punching or machining processes.Additional features, such as alignment features or a lip to provide areduced aperture may be used on this part in order to assemble the partwith a second housing member (6) of the required geometry.

The metal cylinder (4) may be attached by welding, soldering, gluing,crimping or other attachment processes, which can be selected for theircost and hermeticity properties. In the case of welding, resistancewelding, electron-beam welding, laser welding, or other weldingprocesses can be used as appropriate for the size and geometry of thepackage.

The fact that this process is not performed in an inert atmosphere or avacuum, as it would be in a conventional metal can vacuum assemblyprocess, means that a wider range of processes can be used and theassembly cost can be reduced. Whilst the above step does not require aninert atmosphere or a vacuum, it may be desirable to fabricate thehermetic enclosure and attach the electronic device under clean-roomconditions to avoid particulate contamination of the device. Thissub-assembly, along with the other package components, can then bestored in under controlled conditions, at an elevated temperature untilrequired. This reduces the need for time-consuming bake-out processes atthe time of assembly.

The next assembly step is performed under a controlled atmosphere. Thecontrolled atmosphere may comprise an inert gas, for example nitrogen orargon. Additionally, or alternatively, the controlled atmosphere maycomprise a vacuum. Herein, the term vacuum shall refer to a decrease ofpressure below normal ambient atmospheric pressure. Note that thecontrolled atmosphere under which this next assembly step is performedis not merely a clean-room environment as used above for fabricating thehermetic enclosure.

The second housing member (6) is pressed down onto the seal (16), inthis case an indium sealing ring. Location features allow this assemblyprocess to be performed with high accuracy and at low cost.

The second housing member (6) may be designed to slide over the firsthousing member (4), or vice versa, and to form an interference fittherewith. In this case, no additional means are required to hold theenclosure together.

The interference fit itself may be used to provide a seal in this case.Alternatively, where the housing members (4, 6) comprise metalcylinders, the second housing member (6) may be heated prior to assemblyand allowed to cool and contract over the first housing member (4), thusimproving the seal. Alternatively, where the housing members (4, 6)comprise metal cylinders, the friction due to the second housing member(6) being moved over the first housing member (4) can be used to produceheating and produce a friction welded seal or to melt a pre-placed massof solder, referred to herein as frictional soldering.

In a preferred implementation the seal (16) consists of indium solder,although, as mentioned above, other sealing methods can be used.

The above method of forming the hermetic enclosure is advantageous inthat only an alignment and compression operation must be performed undera controlled atmosphere, thus simplifying the manufacture. The low partcount and simple assembly procedure enables a very low cost hermeticenclosure to be manufactured. Furthermore, the fabrication process usesa low temperature sealing step, which has several advantages. Firstly,there is no outgassing during the sealing step. Secondly, materials ofdiffering thermal expansion coefficients can be joined. Thirdly,materials subject to damage by heating or thermal shock can be joined.

Where the enclosure is evacuated to a low pressure, getters may beincluded in the package to remove gases which accumulate due to leakageor outgassing over a period of several years, thereby maintaining a highlevel of vacuum over a long period of time.

In addition, or alternatively, the internal volume of the hermeticallysealed enclosure (2) is arranged to be as large as possible for a givenapplication so as to mitigate the effects of leakage and outgassing onthe vacuum level within the enclosure. A useable vacuum level can besustained over a long period of time by maximising the internal volumeof the hermetically sealed enclosure (2) as suggested above. Forexample, the enclosure may be arranged to exhibit an initial internalpressure of 1 Pascal, degrading to a pressure of 10-100 Pascal over thedesign life of the device (referred to below as the end-of-lifepressure).

Equally, the initial and end-of-life pressures within enclosure may bespecified as design parameters at the outset, in which case the internalvolume of the enclosure can be calculated and optimised such that thepressure within the enclosure at the end of the design life does notexceed the design specification. The calculation requires anappreciation of the outgassing and leakage mechanisms at work within theenclosure, both of which would be familiar to the skilled person.Typically, the end-of-life pressure shall not exceed 100 Pascal, 80Pascal, 60 Pascal, 40 Pascal, 20 Pascal, or 10 Pascal depending on theparticular application.

Typically, the hermetically sealed package will have an internal volumegreater than 0.5 cm³ (cubic centimetres), 0.75 cm³ or 1 cm³. The maximuminternal volume of the hermetically sealed package will be less than 2cm³, 1.75 cm³, 1.5 cm³ or 1.25 cm³. The design life of the device isanticipated to be in excess of 10 years.

Referring now to FIG. 2, the second housing member (6) may performadditional functions, such as providing an aperture for electromagneticradiation to enter or leave. Alternatively, the second housing member(6) may hold one or more additional optical components in place.

In this embodiment of the present invention, the second housing member(6) comprises an optical element (20) providing a window or lens totransmit electromagnetic radiation in a preferred frequency range. Forexample, the optical element (20) may take the form of an infra-redwindow or lens made from a chalcogenide glass material such as the oneof the AMTIR materials manufactured by Amorphous Materials or one of theGASIR materials manufactured by Umicore. These lenses have the advantageof low manufacturing cost. Alternatively, the optical element (20) maytake the form of an infra-red window or lens made from silicon orgermanium.

The use of a lens integral to the second housing member (6) enableselectromagnetic radiation to be focussed onto the electronic device (8)housed within the hermetically sealed enclosure (2), and obviatesexternal optical components.

The electronic device (8) may comprise an optical detector, for examplean optical detector sensitive to electromagnetic radiation havingwavelengths in the visible spectrum, or a microbolometer detectorsensitive to electromagnetic radiation having wavelengths in theinfrared spectrum (for example thermal radiation). In particular, theelectronic device may comprise a multi-element detector, in which casethe integral lens enables a complete imaging system to be achievedwithin the hermetically sealed enclosure (2).

The lens may comprise a simple lens having a single element, a compoundlens having a plurality of elements, a fresnel lens, or an array oflenses, for example an array of microlenses.

The optical element (20) may be metallised at the edges to enable awider range of vacuum sealing processes, for example solderingprocesses. In the case of metallised optical element (20), a solderingprocess may be used to attach the optical element (20) to the secondhousing member (6) prior to assembling the first and second housingmembers (4, 6) as described above. Alternatively, or in addition, a lowtemperature soldering process may be incorporated into the sealing stepsperformed in a controlled atmosphere. The soldering process createsnegligible outgassing and so may be used without compromising the purityof the controlled atmosphere within the hermetic enclosure (2) or thelevel of the vacuum therein.

This versatility with regard to the choice of sealing processes makesthe invention useful in a wide range of applications, since the sealingprocess can be selected in order to obtain the required hermeticity fora given application.

FIG. 3 shows an exploded schematic representation of an hermeticenclosure according to the present invention. Elements in common withFIGS. 1 and 2 are given like reference numerals.

FIG. 4 shows an alternative embodiment of the present invention in whichthe optical element (20) is used in combination with an additionaloptical element (22). In this case, the first optical element (20)comprises an infrared lens or window made from silicon or germanium. Asilicon or germanium window has improved mechanical andthermo-mechanical properties compared to the chalcogenide glass lens.

The additional optical element (22) may comprise a chalcogenide glasslens glued in an accurately-defined location above the first opticalelement (20), the location being defined by location feature(s) in thesecond housing member (6). Hence, the relatively fragile lens (22) isnot directly exposed to the vacuum or to other mechanical and thermalstresses which may be involved in the vacuum sealing process. As above,although an indium seal is illustrated in this case, the first opticalelement (20) may be metallised at the edges to enable a wider range ofvacuum sealing processes, for example soldering processes.

Whilst in the foregoing embodiments the components are largely made ofmetal, the components may alternatively or additionally be made ofceramic (for example, using a ceramic chip carrier in place of the metaltransistor can header), glass, plastic or other suitable materials.

In particular, the hermetically sealed enclosure of the presentinvention may be used to house thermal detectors, for examplemicrobolometer arrays.

1. A method of hermetically packaging an electronic device, in anenclosure comprising mutually inter-engageable first and second housingmembers, comprising the steps of (i) securing the electronic device tothe first housing member, (ii) engaging the first and second housingmembers such that an hermetic seal is provided there between, whereinthe engagement step is performed in a controlled atmosphere.
 2. A methodaccording to claim 1 wherein the first housing member comprises a baseportion, to which the electronic device is secured during the securingstep, and an engagement portion adapted to engage with the secondhousing member, and wherein the method further comprises the step ofattaching the base portion to the engagement portion prior to engagingthe first and second housing members.
 3. A method according to claim 1wherein the first and second housing members are adapted to inter-engageto form an interference fit there between, said interference fitproviding the hermetic seal.
 4. A method according to claim 1 whereinthe enclosure further comprises a seal interposed between the first andsecond housing members, said seal providing the hermetic seal.
 5. Amethod according to claim 4 wherein the seal comprises at least one of ametal, a eutectic alloy, an elastomer and an adhesive.
 6. A methodaccording to claim 5 wherein the seal comprises an indium seal.
 7. Amethod according to claim 5 wherein the seal comprises a compressibleelastomeric ring.
 8. A method according to claim 4 and furthercomprising the intermediate step of applying the seal to at least one ofthe first and second housing members prior to engaging the first andsecond housing members.
 9. A method according to claim 4 wherein theenclosure further comprises a spacer disposed adjacent the seal so as topreclude over-compression of the seal during the engagement step.
 10. Amethod according to claim 4 wherein the enclosure comprises a retainerdisposed adjacent the seal so as to retain the seal.
 11. A methodaccording to claim 1 wherein the second housing member comprises a firstsubstantially transmissive optical element and an engagement portionadapted to engage with the first housing member.
 12. A method accordingto claim 11 wherein the hermetic seal is provided between the firsthousing member and the first optical element via a seal.
 13. A methodaccording to claim 11 wherein the second housing member is adapted toreceive a second substantially transmissive optical element.
 14. Amethod according to claim 1 wherein the controlled atmosphere comprisesan inert gas.
 15. A method according to claim 14 wherein the inert gascomprises at least one of nitrogen and argon.
 16. A method according toclaim 1 wherein the controlled atmosphere comprises a vacuum.
 17. Amethod according to claim 1 wherein the step of engaging the first andsecond housing members includes the step of bonding said housingmembers.
 18. A method according to claim 17 wherein the bonding stepcomprises one of friction welding and friction soldering.
 19. A methodaccording to claim 1 wherein the first and second housing memberscomprise metal cylinders having a substantially circular cross section.20. (canceled)
 21. An electronic device comprising an electronicelement, a first housing member, and a second housing member, the firstand second housing members having an engagement hermetic seal therebetween so as to define around the electronic element an hermeticenclosure having a controlled atmosphere within.
 22. An electronicdevice comprising an electronic element, a first housing member, and asecond housing member, the first and second housing members defining anhermetic enclosure, wherein the electronic element is located within thehermetic enclosure and wherein the hermetic enclosure is formed byengaging the first and second housing members in a controlled atmospheresuch that an engagement hermetic seal is provided there between.
 23. Anelectronic device according to claim 21 wherein the engagement hermeticseal comprises an interference seal between the first and second housingmembers.
 24. An electronic device according to claim 21 wherein theengagement hermetic seal comprises a friction weld between the first andsecond housing members.
 25. An electronic device according to claim 21and further comprising a seal interposed between the first and secondhousing members, said seal providing the engagement hermetic seal. 26.An electronic device according to claim 25 wherein the first and secondhousing members are held in engagement by an interference fit therebetween.
 27. An electronic device according to claim 25 wherein the sealcomprises at least one of a metal, a eutectic alloy, an elastomer and anadhesive.
 28. An electronic device according to claim 27 wherein theseal comprises an indium seal.
 29. An electronic device according toclaim 27 wherein the seal comprises a compressible elastomeric ring. 30.An electronic device according to claim 25 wherein the enclosure furthercomprises a spacer disposed adjacent the seal so as to precludeover-compression of the seal.
 31. An electronic device according toclaim 25 wherein the enclosure comprises a retainer disposed adjacentthe seal so as to retain the seal.
 32. An electronic device according toclaim 21 wherein the second housing member comprises a firstsubstantially transmissive optical element and an engagement portionadapted to engage with the first housing member.
 33. An electronicdevice according to claim 32 wherein the first optical element comprisesa lens.
 34. An electronic device according to claim 32 wherein the firstoptical element comprises at least one of chalcogenide glass, siliconand germanium.
 35. An electronic device according to claim 32 whereinthe engagement hermetic seal is provided between the first housingmember and the first optical element via a seal.
 36. An electronicdevice according to claim 32 wherein the second housing member includesa second substantially transmissive optical element.
 37. An electronicdevice according to claim 36 wherein the second optical elementcomprises chalcogenide glass.
 38. An electronic device according toclaim 21 wherein the controlled atmosphere comprises an inert gas. 39.An electronic device according to claim 38 wherein the inert gascomprises at least one of nitrogen and argon.
 40. An electronic deviceaccording to claim 21 wherein the controlled atmosphere comprises avacuum. 41-55. (canceled)
 56. An electronic device having a thermaldetector housed within an hermetic enclosure comprising mutuallyinter-engaged first and second housing members, the second housingmember comprising a first substantially transmissive optical element;wherein said housing members enclose the thermal detector within acontrolled atmosphere and provide an engagement hermetic seal aroundsaid thermal detector.